It has said that, without the supply of the antiviral drug Tamiflu, made by Swiss drugs firm Roche, an outbreak could kill 50,000 in the UK.

The medication works by reducing the symptoms and the risk of a carrier passing on the virus, at a cost of £180m. It is hoped distributing the drug could reduce demand for acute hospital beds by up to 50%.

Antiviral drugs are most effective if they are given within 48 hours of a patient showing symptoms.

The UK supply will be enough for a quarter of the population - the World Health Organization's recommended level.

Health officials will be the first in line for the drugs, with the remainder being handed out to whichever part of the population is deemed most at risk.

This latest advice, which will be sent out to health organisations, advises on the storage and distribution of the drugs.

'So many unknowns'

However, they will not be sent out to doctors until the World Health Organization's flu pandemic alert level has reached pandemic level.

This would occur at around level six. The current alert level is 0.3.

So far, over 50 people are confirmed to have died from bird flu in South East Asia.

The government guidance sets out how to store the drugs safely, but where they will be easily accessible 24-hours a day in the event of a pandemic.

It suggests cases of flu could be diagnosed over the telephone, or that nurses and pharmacists could be used to prescribe the drugs.

The Department of Health said it was likely the virus would affect some groups of people more than others.

But, because any pandemic is likely to be caused by a strain of flu not seen before, it is not yet clear who that would be.

The government has published initial advice which says drugs should be given to:

Those with an acute flu-like illness

Those with a fever of over 38°Celsius (100.4°Farenheit)

Those who have had symptoms for two days or less

A powdered form of the drug will be available for children aged up to five. Normally they are given a liquid form, but this has a limited shelf-life.

If there is more demand for the drug than the quantity available, groups known to be vulnerable to flu viruses will be given priority, such as people over 65 and diabetics and the elderly.

Dr Michael Dixon, chairman of the NHS Alliance, said he felt the government had achieved a balance between being prepared, and not panicking people.

He told the BBC News website: "There are so many unknowns about how a pandemic would develop.

"So this is good, in that it's trying to hone down the unknowns and give people some sort of a framework, so when it happens we're not surprised.